1. Field of the Invention
The present invention relates to an image forming apparatus capable of selectively forming a multi-color image and a mono-color image.
2. Description of the Prior Art
In the image forming apparatus of this kind of the prior art such as a printer or a PPC reproducing machine, there is used as an image retainer a highly sensitive photosensitive member of selenium-tellurium or selenium-arsenic group, or an organic photosensitive member. However, the photosensitive member of this kind has an optical fatigue. Even after a latent image formed by a first image exposure has been developed, transferred and cleaned and then a uniform charging for forming a second latent image has been accomplished, the first latent image remains as a optical fatigue (a so-called "optical memory"). This causes a problem that a ghost image appears on a subsequent transfer paper, especially when a mono-color image formation adopts a demand system so as to shorten the time period for the image formation.
As a means for solving this problem, there is accomplished a so-called exposure before charging, in which the optical fatigue resulting from the previous image exposure is masked by effecting a uniform exposure before the uniform charging of the image retainer to give a uniform optical fatigue.
FIGS. 7A to 7C are diagrams for explaining changes in the surface potential of an image retainer. FIG. 7 A represents a case in which the exposure before charging is not accomplished. In FIG. 7A , letter (a) indicates the potential of the image exposure, the optical fatigue of which remains as a potential (b) even after a second uniform charging and as a potential (c) even after a third uniform charging. Where a demand system is adopted in a mono-color mode, a position in which a latent image is formed will shift when in a continuous reproducing operation so that the potentials (b) and (c) cause ghost images.
FIG. 7B represents a case of the exposure before charging and shows that the potentials (b) and (c) disappear.
FIG. 7C represents a case of the multi-color mode and shows that since the toner stays stuck until the toner image of a final color is formed on the image retainer, the potential is more smooth than when no toner sticks.
The hatching indicates that the potential is raised as a result of the stuck toner.
The exposure before charging can well exhibit its effect for the mono-color image formation to prevent a ghost image from being formed. In the case of the multi-color image formation, however, the toner image of a first color is subjected to an exposure before charging, when a latent image of a second color is to be formed after completion of the toner image of the first color. Therefore, the amount of exposure differs between the different portions of the image retainer surface, where the toner sticks and not. This causes a difference in the amounts of charge elimination and accordingly in the potentials. As a result, there arises a phenomenon that the charged toner is influenced by an electric field generated by the potential difference so that it scatters.
This discourages the exposure before charging from being used in the case of the multi-color formation, in which toner images of individual colors are to be transferred by one operation after they have been formed on the image retainer.
Here in the case of the multi-color image formation, the toner images are not transferred for each color but have their toners retained as they are. Upon formation of latent image of second or later colors, therefore, the potential of the surface of the image retainer is relatively smoothed, as shown in FIG. 7 C , by the action of a scorotron. As a result, the multi-color image forming apparatus is not usually equipped with means for exposing before charging.
It is, however, the current practice to give the multi-color image forming apparatus a mono-color image forming function. The mono-color image formation frequently adopts a demand system so as to shorten the time period for continuous reproductions.
This involves the aforementioned problem that the ghost image is liable to form. Irrespective of the demand system, there naturally arises the problem that the ghost image is formed even in the case a different image is formed by changing a document shortly after the end of the previous image formation.